EZTwain Pro 3.0 - Using from National Instruments LabView™
Overview 
Download Licensing Features Code Wizard Stories User Guide
We are not qualified to offer specific advice on doing this, but several EZTwain users have successfully used EZTwain from LabView. National Instruments have greatly expanded their on-line material on calling external DLLs, this seems to be the main page:
National Instruments - Developer Zone - LabVIEW - Integrating DLLs
This
article covers some specifics of parameter types:
An
Overview of Accessing DLLs or Shared Libraries from LabVIEW
Key
points:
All EZTwain functions use the stdcall calling convention,
not the 'C' convention.
The library name is Eztwain3.dll
See also: Writing Win32 Dynamic Link Libraries (DLLs) and Calling Them from LabVIEW
See also this small example program that calls a function in a DLL: Getting the "User Name"